Retirement tax questions

Wife and I are at $250,000 combined and I am 56 years old.

In 2021, I opened a backdoor Roth, by contributing $7,000 after-tax dollars into my traditional IRA, and then immediately opening a new Roth and funding it $7,000 from the traditional.

I did not realize there was a penalty, and so a penalty of about $420 has been added to my tax bill.   But I think I can move the $7,000 from Roth back into traditional, and thus avoid the penalty?

Vanguard is not letting me do this, and says it's not allowed to move Roth money into a traditional.  So I'm not sure the best way to handle this and avoid $420 fee.

Any ideas please, or am I getting something wrong?